Hey A.,
I am a big fan of cosleeping too! But it is not for everyone and no lie, it is hard to wean from once started.
But since you had not had this issue with your other children, have you considered acid reflux. My little one was a poor sleeper for 2 years until the reflux was under control and we started treatment early. Reasons for acid reflux can be none at all, to bottle feeding and especially preterm babies.
So if this baby was earlier than due date, or you are bottle feeding this one and breastfed the others, or again no reason at all.... talk to your MD about acid reflux. Sleep disturbance is common with reflux, as your little one is lying down and therefore less gravity to keep the refluxing down, so all the acid is rushing up their throat and the painful burning can be enough to wake them. The easily falling back to sleep, suggest he is tired and not meaning to wake.
Things to look for but not every baby will have with reflux, are decreased appetite, food aversion, smelly spit up, more frequent spit up, vomiting, distruptive sleep patterns, poor weight gain and possible failure to thrive. Look up acid reflux to see more as these are just the ones I recall.
In our case my little one was 110th percentile for weight but they time they aggressive began treating she was vomiting everything that went down and still gaining in height and weight. They wouldn't beleive me how much she puked.... so I just started feeding her in the waiting rooms and then letting her puke in their office. It took the witnessing of the event to understand I was not just a paranoid first timer.
So please know, NOT all babies with reflux will look like they have it. My child had a severe case and honestly should have been severely underweight. Every baby is different, but do trust your mommy instincts.
And for your sake, I do hope it is something simple and not reflux. But more importantly many wishes and prayers you all get needed sleep and stay well.
-MB